This invention relates to peptide-based compounds having light-emitting moieties. Peptides may be chemically linked with detectable xe2x80x9clabelsxe2x80x9d and used, for example, to monitor peptide, cytokine, drug, and hormone receptors at the cellular level. Typically, the labeled peptide is placed in contact with a tissue or cell culture where it binds to an available receptor. Once bound, the label is detected, allowing properties such as receptor distribution or receptor binding kinetics to be monitored.
Peptides are typically labeled with radioactive elements such as 125I or 3H. In this case, emission of high-energy radioactive particles is monitored using standard (xcex3-ray detectors, thereby allowing detection of the label. While detection techniques for 125I and 3H are well-known, radioactive compounds by nature have limited half lives, and are often both toxic and expensive. Moreover, current detection technology makes it difficult or impossible to detect radioactive probes in real-time, thereby precluding study of kinetic processes.
Vasoactive intestinal peptide (VIP) is a 28 amino acid peptide, first isolated in the lung and intestine, but also found in tissues of the brain, liver, pancreas, smooth muscle and lymphocytes. VIP is a particularly desirable peptide to label and use to monitor cell receptors, as this compound exhibits multiple biological roles including vasodilation, electrolyte secretion, modulation of immune function and neurotransmission. In particular, VIP is involved in the regulation of the cardiovascular system by exerting vasodilation, hypotension, positive chronotropic and ionotropic effects.
The present invention provides a compound containing a VIP peptide and a light-emitting moiety that is both biologically active and optically detectable. The peptide is chemically attached to the light-emitting moiety at an amino acid position that is not involved in binding to the peptide receptor. In this way, the peptide""s affinity for the binding site is not significantly decreased, and the compound retains high biological activity and can be easily detected using standard optical means.
In general, in one aspect, the invention provides a biologically active compound of the formula: 
where R1 is a light-emitting moiety and R2 is a VIP-based peptide and fragment, derivative or analog thereof. The peptide is linked at an amino acid position to (Cxe2x80x94X) which, in turn, is selected from the group including Cxe2x95x90O, Cxe2x95x90S, CH(OH), Cxe2x95x90Cxe2x95x90O, Cxe2x95x90NH, CH2, CH(OR), CH(NR), CH(R), CR3R4, and C(OR3)OR4 where R, R3, and R4 are alkyl moieties or substituted alkyl moieties. Preferably, the compound exhibits substantial biological activity in the presence of receptors having affinities for VIP-based peptides. The compound may also be in the form of a pharmaceutically acceptable salt or complex thereof. Preferably, the fifteenth amino acid position which is the lysine 15 amino acid residue is bound to (Cxe2x80x94X) is included of the VIP-based peptide.
In other preferred embodiments, the VIP-based peptide includes the amino acid sequence His-Ser-Asp-Ala-Val-Phe-Thr-Asp-Asn-Tyr-Thr-Arg-Leu-Arg-Lys-Gln-Met-Ala-Val-Lys-Lys-Tyr-Leu-Asn-Ser-Ile-Leu-Asn. (SEQ. ID NO: 1) and the Lys 15 residue is attached to the (Cxe2x80x94X) moiety. The lysine residue preferably is chemically bound to the (Cxe2x80x94X) moiety through the xcex5 amine group of the lysine residue. In still other preferred embodiments, the (Cxe2x80x94X) bond is either Cxe2x95x90O or Cxe2x95x90S. In other preferred embodiments, the peptide may be amidated at the C-terminus.
In other preferred embodiments, the light-emitting moiety (R1) is selected from the group including 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, fluorescein, FTC, Texas red, phycoerythrin, rhodamine, carboxytetramethylrhodamine, 4xe2x80x26-diamidino-2-phenylindole (DAPI), indopyras dyes, Cascade blue, coumarins, nitrobenzofurazane (NBD), Lucifer Yellow, propidium iodide, CY3, CY5, CY9, dinitrophenol (DNP), lanthanide cryptates, lanthanide chelates, non-fluorescent dialdehydes (OPA, NDA, ADA, ATTOTAG reagents from Molecular Probes) which react with primary amines (N-term lysine) in the presence of a nucleophile (i.e. CN) to form fluorescent isoindoles, dansyl dyes fluorescamine and dabcyl chloride, 5-((((2-iodoacetyl)amino)ethyl)amino)naphthalene-1-sulfonic acid, long lifetime dyes comprised of metal-ligand complexes (MLC) which consist of a metal center (Ru, Re, Os) and organic or inorganic ligands complexed to the metal such as [Ru(bpy)3]2+ and [Ru(bpy)2(dcbpy)], and the like and derivatives thereof. The light-emitting moiety can be attached to the peptide by reaction of a reactive side group (of the light-emitting moiety) with the N-terminus amino acid of VIP. Suitable linking moieties include, by way of example only, indoacetamide, maleimide, isothiocyanate, succinimidyl ester, sulfonyl halide, aldehydes, glyoxal, hydrazine and derivatives thereof.
By xe2x80x9cVIP-based compoundxe2x80x9d is meant a peptide which includes VIP, fragments of VIP, derivatives or analogs thereof. VIP-based peptides may be peptides whose sequences differ from VIP""s wild-type sequence by only conservative amino acid substitutions. For example, one amino acid may be substituted for another with similar characteristics (e.g., valine for glycine, arginine for lysine, etc.) or by one or more non-conservative amino acid substitutions, deletions, or insertions which do not abolish the peptide""s biological activity. Other useful modifications include those which increase VIP""s stability. The peptide may contain, for example, one or more non-peptide bonds (which replace a corresponding peptide bond) or D-amino acids in the peptide sequence. Additionally, the C-terminus carboxylic acid group may be modified to increase peptide stability. For example, as described above, the C-terminus may be amidated or otherwise derivatized to reduce the peptide susceptibility to degradation.
In all cases, by xe2x80x9csubstantially biologically activexe2x80x9d is meant the compound binds to a receptor having an affinity IC50 value for the compound which is no more than 15 times, more preferably no more than 10 times and most preferably equal to or less than that of the corresponding unlabeled peptide. Receptor affinity in this case can be determined using known methods, such as methods involving competitive binding of radioactively labeled peptides or by using known methods of fluorescence polarization or other known fluorescence technique for measuring the Kd for the receptor/peptide interaction.
By xe2x80x9clowxe2x80x9d or xe2x80x9cnoxe2x80x9d biological activity or xe2x80x9cbiologically inactivexe2x80x9d is meant biological activities less than 1.0% of the biological activity of R2xe2x80x94H in the presence of a receptor having affinity for VIP.
The above-identified compound is useful in the labeling of cell receptor sites, cell sorting, flow cytometry and performing fluoroimmunoassays. In another aspect, the invention provides a method for labeling a receptor having an affinity for a VIP-based peptide by contacting the receptor with one or more of the compounds described above. Cell receptor sites, can be imaged by contacting candidate cell receptor sites with the compound of the invention, and then detecting the bound compounds as an indication of the cell receptor sites. Cell sorting can be performed by contacting a population of cells with compound and isolating cells bound to the compound. Flow cytometry can be performed by contacting a population of cells with the compound and detecting cells bearing receptors on their surfaces by detecting cells bound to the compound.
The invention has many advantages. In a general sense, peptide-containing compounds which retain their biological activity after being labeled with light-emitting moieties have a wide variety of biological applications. Such compounds can be used to identify, visualize, quantify, target and select receptors on cells and tissues both in vitro and in vivo. These compounds may be used in place of more conventional labeled peptides, such a 125I radiolabeled peptides. Radiolabeled compounds are often toxic, environmentally hazardous, chemically unstable and have, by the nature of the radioactive decay rate, relatively short lifetimes. In contrast, fluorescently-labeled VIP is relatively safe and non-toxic, thereby allowing it to be synthesized and used without employing special laboratory procedures. Similarly, following use, fluorescent VIP may be easily disposed, whereas disposal of radioactive compounds is both time-consuming and costly. In addition, fluorescent markers for VIP receptors are stable and may be stored for extensive periods of time without undergoing extensive degradation.
Use of VIP in the labeled compound is also advantageous. As described above, VIP exhibits biological activity in various and is involved in regulation of the cardiovascular system. In addition, this peptide has a relatively simple structure (28 amino acids) and can be synthesized and isolated with standard, well-known techniques.
During typical experiments, fluorescent markers for VIP receptors emit optical signals, and thus may be monitored by eye or with the aid of external optical detectors. In this way, the fluorescent peptides obviate the need for secondary detection steps sometimes used for radiolabeled compounds or incubation with secondary labeled compounds. Detection of optical radiation is, in general, relatively simple and cost-effective compared to detection of radioactive particles (e.g., xcex3-particles); conventional charge-coupled device (CCDs) or light-sensitive cameras can therefore be used without modification for this application.
In addition, because of their high optical emission rates and well-characterized emission cross sections, fluorescent markers attached to VIP receptors can be used for real-time, quantified imaging of a number of dynamic biological phenomena, such as kinetics associated with receptor binding. The compounds can also be used for static processes, such as monitoring peptide distribution within a cell. VIP receptors marked with fluorescent peptides may also be used in flow cytometry, confocal microscopy, fluorescence polarization spectroscopy, and any other techniques exploiting the optical detection of fluorescence or photoluminescence.
Other advantages and features of the invention will become apparent from the following detailed description, and from the claims.